Help me understand aero: rear corner radii, yes or no?

[MetroMPG]

This is one question I've never been able to answer to my satisfaction:

Regarding transitions from the side of the rear quarter panels of a vehicle to the rear-facing areas (ie the plane that the licence plate is on), what's better for lower Cd: transitions constructed with generous radii, or transitions that are sharp?

The reason I'm confused is because images I have seen conflict with each other and what I have read. What I see on a large number of ultra-sleek concept cars are side/rear transitions which look literally "chopped" - sharp 90 degree corners where the rear quarter panels meet the rear-facing bits.

EG: VW 1-liter car is a prime example; the PNGV cars; Ford Probe V.

Yet I have read (Barnhard, Road Vehicle Aerodynamic Design) that side-to-rear transitions should be generously rounded for reduced drag. With the caveat that generously rounded side-rear transitions are also less stable in cross-winds than sharp rear angles.

EG: several low drag production cars, notably the Insight, and the EV1, had generously rounded side-rear corners - the bumper covers below the tail lights anyway. Which makes me wonder if, because these are production vehicles, some concession to user-friendliness was made over aerodynamics (ie: round bumpers seem less likely to be damaged in minor scrapes than bumpers which have a sharp 90 degree corner).

Does anyone have any credible information or reasoning to help answer this?

I find the apparent "contradictions" on this design element between these otherwise very slick cars a bit confusing.